Running catphotomfit

To perform a simple fit to a set of standard stars type:

catphotomfit

Your catalogue of standard stars should contain an air mass for each star. catphotomfit will determine the transformation coefficients, display them together with the residuals and write the coefficients to a file. If your catalogue contains a column of observed zenith distances rather than air masses then type:

catphotomfit  zenithdist=true

See Section  for details of how the air mass is calculated from the zenith distance. If some of the transformation coefficients are fixed (that is, you know them beforehand) type:

catphotomfit  fixed=true

You will be prompted for details of which coefficients are fixed and their values. If all the coefficients are fixed then obviously no fit is made. However, the residuals are still computed and listed and a file of transformation coefficients is written. To suppress the listing of residuals type:

catphotomfit  resid=false

These options can be combined. Thus, to read a catalogue containing zenith distances rather than air masses and fix some of the transformation coefficients type:

catphotomfit  zenithdist=true  fixed=true

You then answer a series of prompts. All the possible prompts are listed below, identified by the corresponding ADAM parameter name. All the prompts will not appear in a given run. For example, none of the prompts FZEROP, ZEROP, FATMOS or ATMOS appear if none of the transformation coefficients are fixed.

FZEROP

Specify whether the zero point is fixed. The possible replies are:

TRUE

the zero point is fixed,

FALSE

the zero point is not fixed.

ZEROP

Enter the value of the fixed zero point.

FATMOS

Specify whether the atmospheric extinction is fixed. The possible replies are:

TRUE

the atmospheric extinction is fixed,

FALSE

the atmospheric extinction is not fixed.

ATMOS

Enter the value of the fixed atmospheric extinction.

INSCON

Enter the arbitrary constant previously added to the instrumental magnitudes.

CATALOGUE

Enter the name of the catalogue containing the standard and catalogue magnitudes.

NAME

Enter the name of the column containing names of the standard stars. The special value `NONE' indicates that a column of star names is not required.

INCLUDE

Enter the name of the column of `include in the fit' flags for the standard stars. The special value `ALL' indicates that all the stars are to be included in the fit.

CATMAG

Enter the name of the column or expression holding the standard or catalogue magnitudes.

INSMAG

Enter the name of the column or expression holding the instrumental magnitudes.

AIRMASS

Enter the name of the column or expression holding the air masses.

ZENDST

Enter the name of the column or expression holding the observed zenith distances.

FILNME

Enter the name of the file which is to contain the transformation coefficients.

Figure: Example output from catphotomfit

Figure  shows the output displayed by catphotomfit. The transformation coefficients are self-explanatory. The minimum residual vector length is a measure of the goodness of the fit. The table of residuals is also mostly self-explanatory. The column of star names will be absent if parameter NAME was specified as `NONE'. A `Y' in the `Fit' column indicates that the star was included in the fit. The residuals are defined in the sense:

$$m_{\rm catalogue} - m_{\rm calculated}$$ (10)

The calculated magnitudes and residuals are shown to three places of decimals. This format does not imply that the results are this accurate; the actual accuracy will depend on the data used. It is noteworthy, however, that in the example data the largest residual is only slightly larger than 0.01 magnitude, despite the method ignoring colour corrections.

The bar to the right of the residuals is a simple graphic representation of the absolute size of the residual; the length of the bar is scaled according to the absolute size of the residual for the star. The scaling is such that the largest absolute residual amongst the stars included in the fit is ten asterisks long. Stars which are included in the fit are shown as a row of asterisks (`*'). Stars which are excluded from the fit are shown as a row of dashes (`-'). Because excluded stars will often have larger residuals than the included stars, for excluded stars with residuals larger than the largest included residual a right chevron (`>') is shown beyond the last dash (thus forming an arrow).

For completeness, and to avoid any possible ambiguity, the formula used to compute the standard deviation, $s$, is:

$$s = \sqrt{ \frac{1}{(n - 1)} \sum_{i=1}^{n} (m(i)_{\rm catalogue} - m(i)_{\rm calculated})^2 }$$ (11)

where $n$ is either the number of stars included in the fit or the total number of stars, as appropriate.